Graphene (multilayer) boron nitride heteroepitaxy for electronic device applications

ABSTRACT

Disclosed is a substrate-mediated assembly for graphene structures. According to an embodiment, long-range ordered, multilayer BN(111) films can be formed by atomic layer deposition (ALD) onto a substrate. The subject BN(111) films can then be used to order carbon atoms into a graphene sheet during a carbon deposition process.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 13/242,878filed Sep. 23, 2011, now U.S. Pat. No. 8,338,825, which is a divisionalapplication of U.S. patent application Ser. No. 12/543,053, filed onAug. 18, 2009, now U.S. Pat. No. 8,158,200, which is incorporated byreference in its entirety.

The subject invention was made, in part, with government support fromthe Office of Naval Research contract No, N00014-08-1-1107 through asubcontract from the Texas State University at San Marcos. Thegovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

Graphene is a single atomic layer of graphite, and currently holdsinterest for many electronic and spintronic device applications due tographene's electronic properties, including high mobility, highsaturation velocity, stable crystal structure, and ultrathin layerthickness. To be utilized in electronic and spintronic deviceapplications, graphene must rest on an electronically semiconducting orinsulating substrate.

Current methods of producing graphene involve either growing graphene ona transition metal substrate and then transferring a single graphenesheet onto an insulating substrate, or growing graphene by thermalevaporation of silicon from bulk SiC(0001) substrates.

In the first method, the graphene growth on the transition metalsubstrate is typically performed by thermal decomposition of ahydrocarbon precursor, or by formation of highly oriented pyroliticgraphite (a commercial substance), followed by chemical or manualexfoliation of a single graphene sheet and manual placement on aninsulating substrate, such as SiO₂. These methods tend to be unpracticalfor large-scale production of electronic devices with consistentelectronic properties, including graphene-substrate contactcharacteristics.

In the second method, the graphene remains on the semiconducting SiCsubstrate after the thermal of evaporation of the silicon from the SiCsubstrate. The transfer of graphene sheets grown by this second methodis not practical. In addition, the growth process occurs at temperaturesgreater than 1500 K and does not allow for growth or integration withany device materials other than SiC—a material with numerous processingproblems for device manufacture.

Recently, boron nitride (BN) has become of interest for its insulatingcharacteristics and potential for use in nanotechnology. Hexagonal phaseBN films may be grown by atomic layer deposition using layer-by-layerdeposition of precursors to give a film thickness controllable to atomicdimensions. Ordered hexagonal (BN111) films 1 atomic layer thick havebeen grown on Ni(111), Cu(111) and Rh(111) or Ru(0001) surfaces bythermal decomposition of borazinc. However, growth of multilayer filmsby current methods is inhibited because the first BN monolayer rendersthe surface chemically inert. Moreover, the chemical structure ofmonolayer films grown by borazine decomposition on certain substratessuch as Ru(0001) and Rh(111) is highly puckered—a “nanomesh”—and may beunsuitable for subsequent growth of ordered films. In addition, althoughcertain monolayer (BN111) and graphene/monolayer (BN111) films have beengrown, the electronic properties of these graphene films are greatlyaffected by electronic interaction through the single atomic layer ofthe graphene to the substrate. Thus, research continues to be conductedto provide useful graphene structures for device applications.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention provide methods for fabricatinggraphene structures that can be utilized for electronic deviceapplications. Carbon atoms can be ordered into a graphene sheet during acarbon deposition process onto a substrate having multilayer BN filmsfabricated in accordance with certain embodiments of the presentinvention. The multilayer BN films can provide an electricallyinsulating structure between the graphene and the substrate. Inaddition, the multilayer BN films can serve as a topographic template toorder carbon atoms into a graphene sheet during a deposition process. Byusing the BN films for ordering the carbon atoms into the graphenesheet, a substrate-mediated assembly is provided.

The growth of graphene sheets on BN films in accordance with embodimentsof the present invention can further enable growth and patterning ofgraphene films on many different conductive substrates where the BNthickness can be precisely controlled to govern electron transportbetween the conductive substrate underneath the BN and the graphenelayer.

According to one embodiment, a method of fabricating electronic devicesincludes graphene heteroepitaxy by substrate mediated assembly onmultilayer BN(111) films with long range order.

In one aspect, (BN111) films are formed on substrates in multiple layersat controllable thicknesses greater than 1 atomic layer.

In another aspect, the subject multilayer (BN111) films are used forsubstrate-mediated assembly of a graphene overlayer.

In accordance with an embodiment of the present invention, large scaleproduction of BN films and graphene structures can be achieved.

In a further embodiment, the subject methods can enable integration ofgraphene/BN devices with other devices.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features will be apparent fromthe description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a representation of a graphene/(multilayer)BN structure inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide a substrate-mediatedassembly for graphene structures. According to an embodiment, long-rangeordered, multilayer (BN111) films can be formed by atomic layerdeposition (ALO) onto a substrate. Because BN films are isostructuraland isoelectronic with graphene, thereby providing a chemically inerttemplate, the (BN111) films are useful for subsequently ordering carbonatoms into a graphene sheet. In particular, the (111) in-plane latticeconstant of (BN111) is 2.50 Å, which is a close lattice match tographene (2.46 Å). Accordingly, in a further embodiment of the presentinvention, the subject (BN111) films can then be used to order carbonatoms into a graphene sheet during a carbon deposition process.

Current CMOS devices are approaching certain physical limits due tocontinual reduction in the sixes of device features, such as the gateoxide thickness and interconnect line width, the wave nature ofelectrons manifested on atomic dimensions, and the fundamental behaviorof electrons in Si. Since graphene is a semi-metal, faster devices canbe fabricated in place of or as a complement to CMOS devices. Inaddition, graphene devices can be implemented for charge-based,spin-based, and even qubits-based device applications.

(BN111) is an insulating material with a direct band gap of 5.97 eV.Therefore, (BN111) can be a useful insulator between the graphene and asubstrate in electronic, optoelectronic and spintronic deviceapplications. In order to produce useful graphene structures for deviceapplications, (BN111) films with controllable thicknesses greater thanone atomic layer (monolayer) are provided.

BN is an electronic insulator and when provided at thicknesses greaterthan one atomic layer, BN can be used to limit electron tunnelingtransport between a metal substrate and graphene. Such graphene/BN/metalstructures can then be patterned by various methods, such as anysuitable method known in the art, for multiple device applications.

According to various embodiments of the present invention, the subjectmultilayer (BN111) films can be grown on a substrate by atomic layerdeposition (ALD) using a boron-halide or organoborane precursor, BX₃(where X=chlorine Cl, bromine Br, iodine I, an alkyl group such as CH₃or any combination of these) and NH₃. In the ALD process, the substrateis exposed alternately to each precursor, with a reaction occurringbetween precursors to form the desired film in a layer-by-layer manner.In this case, the relevant reaction is:BX₃+NH₃→BN(film)+3HX(desorbed).

Multilayer films may be grown on many different substrates by alternateBX₃/NH₃ exposures to the substrate. According to certain embodiments theexposures can be performed at temperatures of about 550 K to about 750 Kand pressure of 0.01-1000 Torr. In a specific embodiment, the pressureconditions are between 0.01 to 10 Torr. After depositing the multilayerBN film, the substrate can be annealed in vacuum or inert atmosphere toa high temperature, such as 1000 K. Typical film thickness growth ratesunder the above condition are ˜1.3 Å per BX₃/NH₃ cycle.

The substrate can be selected to provide a suitable lattice structure toenable the formation of a highly crystalline film (i.e. with long rangeorder) upon performing the annealing process. The substrate can be the(111) surfaces of transition metals. The substrate can be a bulk singlecrystal substrate or an ordered thin film on another substrate, wherethe in-plane lattice constant is between about 3.0 Å and about 2.2 Å.According to certain embodiments, suitable substrates include, but arenot limited to Ru(0001), Pt(111), Pd(111), Ni(111), Nb(111) and Cu(111)single crystals, epitaxial films grown on Al₂O₃(0001), orderedAl₂O₃(111) films grown on suitable NiAl or Ni₃Al single crystalsubstrates, and suitable (111) grains of Ru, Pt, Ni, Nb or Cu filmsgrown as interconnect lines at the interconnect or packaging levels ofCMOS devices. A silicon substrate may be used where a metal film having(111) surfaces is provided for forming the BN layer.

Graphene can then be formed on the (BN111) film by any suitable methodto achieve a graphene/(multilayer) (BN111)/substrate structure. Auniform graphene film can be obtained by exposure of the ordered (BN111)films to carbon. The graphene can be formed by performing a carbondeposition process. For example, the carbon deposition process caninclude, but is not limited to chemical vapor deposition (CVD) atsuitable temperatures (˜1000 K) with, e.g., ethylene, benzene, methaneor other volatile hydrocarbon precursors, evaporation of carbon from agraphite source, or sputter deposition of carbon from a graphite sputtertarget at suitable temperatures (˜1000 K).

For CVD using ethylene, the relevant reaction is:

${\frac{n}{2}H_{2}C} = {{CH}_{2}->{{C_{n}({graphene})} + {n\;{H_{2}.}}}}$

According to one embodiment, the uniform graphene films can be obtainedby exposure of the ordered (BN111) films to C2H4 at pressures of about0.01 Torr to about 1 Torr at 1000 K.

For CVD using benzene, the relevant reaction is:

${\frac{n}{6}C_{6}H_{6}}->{{C_{n}({graphene})} + {\frac{n}{2}{H_{2}.}}}$

It is also contemplated within the scope of the present invention othermethods for making graphene, including growth of graphene monolayersfollowed by manual liftoff/transfer to the substrate having the orderedmultilayer BN films.

FIG. 1 shows a representation of a graphene/(multilayer)BN structure inaccordance with an embodiment of the present invention. As shown in FIG.1, a substrate 100 can be provided with multiple layers of an ordered BNfilm 110, and a graphene film 128 can be formed on the top layer of theBN film 110.

By fabricating the (BN111) films using ALD in accordance with certainembodiments of the present invention, long-range ordered, multilayer(BN111) films can be provided on a substrate. Furthermore, the subjectlong-range ordered (BN111) films are provided in thicknesses greaterthan one monolayer (or atomic layer).

Embodiments of the present invention can be used for a variety ofelectronic device applications, including electronic, optoelectronic andspintronic devices. Graphene/(BN111) structures fabricated in accordancewith embodiments of the present invention can exhibit high electronmobilities and other properties indicative of a 2-dimensional electrongas. In addition, the use of (BN111) films on various metal(111)surfaces allows the growth of graphene films on insulating substrates onmetal(111) films grown on sapphire. In further embodiments, the use ofthe subject ordered BN(111) films allows growth of graphene films onhighly oriented metal films on CMOS devices.

Ordered BN and BC_(x)M layered films fabricated in accordance with anembodiment of the present invention can exhibit direct band gaps tunablebetween ˜2 eV-6 eV. Furthermore, the combination of ordered graphenefilms on ordered BN (or carbon-doped BN) films of variable thicknessaffords multiple potential device applications.

Any reference in this specification to “one embodiment,” “anembodiment,” “example embodiment,” etc., means that a particularfeature, structure, or characteristic described in connection with theembodiment is included in at least one embodiment of the invention. Theappearances of such phrases in various places in the specification arenot necessarily all referring to the same embodiment. Further, when aparticular feature, structure, or characteristic is described inconnection with any embodiment, it is submitted that it is within thepurview of one skilled in the art to utilize or combine such feature,structure, or characteristic in connection with other ones of theembodiments.

Although embodiments have been described with reference to a number ofillustrative embodiments thereof, it should be understood that numerousother modifications and embodiments can be devised by those skilled inthe art that will fall within the spirit and scope of the principles ofthis disclosure. More particularly, various variations and modificationsare possible in the component parts and/or arrangements of the subjectcombination arrangement within the scope of the disclosure, the drawingsand the appended claims. In addition to variations and modifications inthe component parts and/or arrangements, alternative uses will also beapparent to those skilled in the art.

What is claimed is:
 1. A composition of matter, comprising: an orderedfilm of boron nitride (BN) prepared by atomic layer deposition comprisedof multiple layers and having a thickness greater than one atomic layerof hexagonal boron nitride, said ordered film of BN exhibiting at leastone major surface; and a layer of graphene formed on said major surfaceby chemical vapor deposition.
 2. The composition of matter according toclaim 1, wherein said ordered film of BN exhibits a band gap of about 2eV-6 eV.
 3. The composition of matter according to claim 1, wherein saidordered film of BN is positioned on a substrate on a side of saidordered film of BN opposite said at least one major surface.
 4. Thecomposition of matter according to claim 3, wherein the substrate is a(111)-textured transition metal.
 5. The composition of matter accordingto claim 3, wherein the substrate is a bulk single crystal substrate oran ordered thin film on a secondary substrate, the bulk single crystalsubstrate or ordered thin film having in-plane lattice constant betweenabout 3.0 Å and 2.2 Å.
 6. The composition of matter according to claim3, wherein the substrate is a metal interconnection or packaging levelmetal line on a complementary metal oxide semiconductor substrate. 7.The composition of matter according to claim 6, wherein the metalinterconnection or packaging level metal line comprises grains of Ru,Pt, Ni, Nb, or Cu.
 8. The composition of matter according to claim 3,wherein the substrate comprises Ru, Pd, Ni, Cu, Nb, Pt or Al₂O₃.
 9. Thecomposition of matter according to claim 3, wherein the substratecomprises silicon with a metal film provided between the silicon andsaid ordered film of BN.